Giant robots from space battle it out over the course of this movie and its sequel, causing quite a bit of damage along the way. They're so big, in fact, that just walking around causes the ground to shake! But when these robots are done mixing it up, they snap together and fold their way down to smaller sizes and more manageable weights.
So what happens to their mass? A fundamental rule in the universe is that mass cannot be destroyed, so making something smaller doesn't mean it will be lighter in weight! Any Transformer keeping its mass will therefore become very dense: A 100-foot-tall robot compacting down to a 10-foot car would plunge right through the road and into Earth's crust.
That would be amusing to watch but would make endless sequels unlikely.